Peter Sellers as Grand Duchess Gloriana XII / Prime Minister Count Rupert Mountjoy / Tully Bascombe

Jean Seberg as Helen Kokintz

William Hartnell as Will Buckley

David Kossoff as Professor Alfred Kokintz

Leo McKern as Benter

MacDonald Parke as General Snippet

Austin Willis as United States Secretary of Defense

Timothy Bateson as Roger

Monte Landis as Cobbley

Alan Gifford as Air Raid Warden

Colin Gordon as BBC Announcer

Harold Kasket as Pedro

Guy Deghy as Soviet Ambassador (uncredited)

Eve Eden as Spectator (uncredited)

Bill Edwards as Army Captain (uncredited)

Mabel Etherington as Ada - Lady in Waiting (uncredited)

Muriel Greenslade as Lady in Waiting (uncredited)

Fred Haggerty as Fenwickian (uncredited)

Juba Kennerley as Maybury (uncredited)

George Margo as O'Hara (uncredited)

Lionel Murton as American General at the Pentagon (uncredited)

Bill Nagy as U.S. Policeman (uncredited)

Robert O'Neil as Reporter (uncredited)

Pat Ryan as American General (uncredited)

Stuart Saunders as Sergeant (uncredited)

Fred Stroud as Fenwickian (uncredited)

John Tatum as American Admiral (uncredited)

Mavis Villiers as Telephone Operator (uncredited)

Fred Wood as Bird Seller (uncredited)